1. Field of the Invention
The present invention relates to proton beam targets used in contraband detection systems and other applications including medical imaging, and particularly to more durable proton beam target designs and methods of their manufacture.
2. Description of the Prior Art
Systems for detecting nitrogen based elements in contraband materials are fairly well known. These systems basically utilize the irradiation of such materials with gamma rays and the detection of gamma rays emitted or absorbed by the materials after subjecting them to the input gamma rays of specific energy to be preferentially absorbed or to induce fluorescence in the specific elemental material being detected. One technique of such detection is Gamma Resonance Absorption (GRA) analysis. This type of system generally utilizes the effect of gamma ray absorption by the nucleus of the objects being interrogated during irradiation. The concentration of these gamma rays are detected by gamma ray detectors or arrays of detectors and the signals analyzed to determine the concentrations of chemical elements which make up the object being interrogated. These elements are found in explosives or illicit drugs in differing quantities, ratios and concentrations. By calculating and determining the ratios and concentrations, it is possible to identify and differentiate targeted contraband substances.
In such Contraband Detection Systems (CD or CDS), an example of which is shown in FIG. 1(a), a proton beam 10 is generated that is directed to a proton beam target device 12 that generates a gamma ray fan 15 that is directed to a target object 20 such as a rotating baggage container. Such a GRA CDS system is described in U.S. Pat. No. 5,784,430 the whole contents and disclosure of which is incorporated by reference as if fully set forth herein. Such CD systems are distinguishable by the manner in which the proton beam is generated: 1) Electrostatic Accelerator based, and 2) RF Accelerator based. An early form of the Electrostatic Accelerator based CDS comprises a high current (e.g., 10 mA) electrostatic accelerator, a specially designed proton beam target 12 for gamma generation, and a detector such as segmented and arrayed Bismuth Germinate (BGO) detectors 25. The accelerator produces a beam of protons 10, e.g., at energies of about 1.75 MeV, with a very narrow energy spread. As shown in FIG. 1(b), this high energy proton beam is bombarded onto the specially designed target 12 which is coated with a thin film of 13C (of about 1 micron thick) to generate resonant gamma rays 15a at an energy of about 9.17 MeV by the reaction 13C(p,γ)14N and, additionally, generates non-resonant gamma emissions 15b. The resultant gamma rays 15a are preferentially absorbed by 14N in explosives-type contraband. The penetrating power of the gamma rays combined with a tomographic detection scheme allows 3-D images of the total density and select element density in the interrogated cargo/luggage/container to be generated which is then utilized to detect for the presence of concealed explosives utilizing the ratio of resonant to non-resonant absorption thereby providing the ratio of Nitrogen density to total density.
With more particularity, as shown in FIG. 2, one prior art configuration for the Electrostatic Accelerator based CDS proton beam target 50 used in a GRA system, such as described in issued U.S. Pat. No. 6,215,851, the contents and disclosure of which is incorporated by reference herein, consists of a thin film 52 of 13C deposited onto a substrate comprising a layer 54 of suitable high atomic number (Z) material. The high Z material serves as a stopping layer for the energetic protons after they have traversed through the 13C layer 52. The stopping layer must be composed of a material that will not react with the high energy proton beam to produce additional gamma signals which will interfere with the desired 13C resonant gamma emission. The prior art target device 50 has utilized electroplated gold (Au) as the stopping layer 54. The high Z layer also needs to be of a minimal thickness (roughly 20 microns for Au) necessary to fully stop the proton beam without substantially attenuating the gamma signal generated by the 13C layer. This thickness requirement for the stopping layer necessitates its application onto a low Z, high thermal conductivity, cooling support 56 typically fabricated from Cu or Be to allow for adequate heat dissipation from the proton implant zone. Historically, an amorphous 13C layer 52 is deposited onto the Au stopping layer by an e-beam evaporation technique utilizing amorphous 13C powder as a source.
The need for fast inspection time in a new system gives rise to proton beam currents of 10 mA or higher impinging onto the target. Proton beam targets fabricated in this prior art configuration are not able to withstand such high current bombardment. After limited proton exposures the targets show evidence of film blistering and 13C coating delamination.
With respect to the RF Accelerator based CDS, these systems typically comprise an RF accelerator based proton beam generator unit, and may incorporate energy discriminating, nitrogenous liquid scintillators (replacing the Bismuth Germinate (BGO) scintillator detectors of the Electrostatic Accelerator based unit) that can distinguish between photoprotons and photoelectrons produced in gamma rays interactions in the detector. This type of detector is described in U.S. Pat. Nos. 5,040,200 and 5,323,004 the whole contents and disclosure of which is incorporated by reference as if fully set forth herein. The advantage of this type of detector is that it does not require very tight control of the proton beam energy spread thus making it practical to utilize a radio frequency accelerator instead of an electrostatic based design for gamma generation. An RF accelerator based unit is desirable as it provides a simpler system design with higher current output and improved overall system reliability at comparable cost to an electrostatic device.
These “second generation” RF Accelerator based systems also utilize 8 MeV gamma rays in addition to the 9.17 MeV resonant gamma rays for normalization, where the 8 MeV gamma rays are produced from proton beam interaction with a much thicker 13C layer on the target (in excess of fifty (50) microns). This detection scheme and thicker proton beam target contributes to improved contrast in the generated tomographic images. This allows for much better detection of explosives and contraband disguised in sheet form, aunique advantage of the GRA based tomographic detection scheme over conventional X-ray based approaches.
With more particularity, the prior art configuration for the RF Accelerator based CDS proton beam target incorporates a roughly 50 micron thick 13C layer that also must withstand interaction with a high current (e.g., 10 mA) proton beam. In the prior art proton beam target design, a film of amorphous, isotopic 13C would be deposited onto a substrate comprised of a twenty micron thick gold layer electroplated onto a Cu or Be cooling/structural support (as shown in FIG. 2). The one micron thick isotopic 13C layer would be deposited onto the target with e-beam evaporation techniques utilizing a solid (powder) source of 13C. This thin film deposition technique, though, is unsuitable for fabricating 50 micron thick 13C layers. Thick carbon layers, e.g., layers greater than ten microns thick) can be deposited by high temperature chemical vapor deposition (CVD) methods from gas phase hydrocarbon precursors (propane, methane, etc.) to produce what is commonly referred to as pyrolytic carbon. There are several drawbacks to CVD processes for fabricating proton beam targets. Producing a fifty micron thick or greater pyrolytic carbon layer by thermal CVD is at the limit of the technique's process window. Thicker coatings typically delaminate from substrates as a result of internal stress built-up in the film which scales with film thickness. Pyrolytic carbon deposition by CVD also requires substrate temperatures in excess of 1000-1700° C. that limits the range of suitable materials which can be utilized as support substrates. Thermal expansion mismatch between the pyrolytic carbon layer and most metals (such as the standard copper/gold support substrate previously used in GRA systems) result in film delamination either upon substrate cool down after 13C layer deposition or during high current, proton beam exposures in the detection system. Utilization of a CVD process to produce 13C pyrolytic carbon layers are also costly as the isotopic hydrocarbon gas source (13CH4) is expensive and the gas to solid conversation in the process is inefficient with a major portion of the source gas exiting the deposition chamber unreacted as exhaust.
With the on-going threat of terrorism all over the world, the need has come for improved means of detecting contraband materials, including nitrogen and nuclear containing explosives that may be concealed in vehicles such as cars, trucks automobiles, shipping containers, airplanes, etc. This requires the implementation of improved proton beam target devices.